Using smartcards to enable probabilistic transactions on an untrusted device

Abstract

The present invention permits a user to conduct remote transactions without a network while using an untrusted computing device, such as a hand-held personal digital assistant or a laptop computer. The computing device is augmented with a smartcard reader, and the user obtains a smartcard and connects it to the device. This design can be used by an untrusted user to perform financial transactions, such as placing bets on the outcome of a probabilistic computation. Protocols are presented for adding (purchasing) or removing (selling) value on the smartcard, again without requiring a network connection. Using the instant protocols, neither the user nor the entity issuing the smartcards can benefit from cheating.

1. A method of adding a value to a cryptographic module, comprising:
 receiving, from an untrusted apparatus, a request to update a value on the cryptographic module;  
 calculating a first authorization code based on the value and a secure cryptographic key;  
 transmitting the first authorization code to the untrusted apparatus in response to the request;  
 receiving a second authorization code from the untrusted apparatus;  
 validating the second authorization code based on the first authorization code and the secure cryptographic key;  
 updating the value on the cryptographic module for use with a probabilistic transaction;  
 generating a first random number;  
 receiving a second random number from the untrusted apparatus, the second random number generated from a number by the untrusted apparatus;  
 transmitting the first random number to the untrusted apparatus after receiving the second random number;  
 receiving the commitment number from the untrusted apparatus after said transmitting the first random number;  
 calculating a third random number to enable the probabilistic transaction with the untrusted apparatus, the third random number called from the first random number and the commitment number; and  
 validating the third random number based on the second random number.  
 
     
     
       2. The method of clam  1 , said calculating the authentication code further comprising:
 calculating the authentication code using a pseudo-random function.  
 
     
     
       3. The method of  claim 1  wherein the authentication code is a secure message authentication code (MAC). 
     
     
       4. The method of  claim 1  wherein the cryptographic module is a smartcard. 
     
     
       5. A method for enabling transactions between a cryptographic module and an untrusted apparatus, comprising:
 generating a first random number;  
 receiving a second random number from an untrusted apparatus, the second random number generated from a commitment of number by the untrusted apparatus;  
 transmitting die first random number to the untrusted apparatus after receiving the second random number;  
 receiving the commitment number from the untrusted apparatus after transmitting the first random number;  
 calculating a third random number to enable a probabilistic transaction with the untrusted apparatus, the third random number calculated from the first random number and the commitment number; and  
 validating the third random number based on the second random number.  
 
     
     
       6. The method of  claim 5 , further comprising:
 generating a first salt random number, and  
 transmitting the first salt random number to the untrusted apparatus prior to said transmitting the first random number, said receiving a commitment number further comprising: 
 receiving a second salt random number from the untrusted apparatus, the second random number generated from the first salt number, the second salt number and the commitment number.  
 
 
     
     
       7. A method for enabling remote transitions with a cryptographic module, comprising:
 selecting a commitment number, generating a first random number based on the commitment number,  
 transmitting the first random number to the cryptographic module for validating the commitment number;  
 receiving a second random number generated by the cryptographic module after transmitting the first random number;  
 transmitting the commitment number to the cryptographic module after receiving the second random number; and  
 calculating a third random number based on the second random number and the commitment number the third random number enabling a probabilistic transaction with the cryptographic module.  
 
     
     
       8. The method of  claim 7 , further comprising:
 receiving a first salt random number generated by the cryptographic module;  
 generating a second salt random number; and  
 transmitting the second salt random number to the cryptographic module after said receiving the first random number, said generating the first random number further comprising: 
 generating the first random number based on the commitment number, the first salt random number and the second salt random number.